Sound recording apparatus



A 30, 1941. J. L. UNDERHILL ETAL 2,263,097

SOUND RECORDING APPARATUS Filed May 28, 1938 25 1 1G. Z. 28 flMPL/r/EnSHUTTER 1 RECTIFIER 3 I'll? 'I'ER n/cRoPHoA/z: J 24 HF fiMPL/F/ER 1 26 IA GflLV/I/VOMETER mm 1 10 a lzwen tors Joseph L. Underhill Lenn .DLmic/B Patented Dec. 30, 1941 SOUND RECORDING APPARATUS Joseph L.Underhill, London, England, and Glenn L. Dimmick, Haddonfield, N. J.,assignors to Radio Corporation of Delaware America, a corporation ofApplication, May 28, 1938, Serial No. 210,592

Claims. (01; 179-4003) This invention relates to an improved soundrecording apparatus and more particularly to sound recording apparatus,for variable density recording with ground noise reduction.

In Dimmick Patents Nos. 2,095,317 and 2,095,-

318, patented October 12, 1937, there are disclosed methods andapparatus for variable density sound recording by what is known as thepenumbra method, wherein an opaque mask produces a graduated intensityin a beam of light and the beam of light is then vibrated acrossa platehaving a slit so positioned as to select anappropriate portion of thebeam which is thereafter directed onto the sensitive film.

In both of the aforesaid patents, ground noise reduction may be applied,and specific apparatus for that purpose is shown in Fig. '7 of thelatterof the patents.

The present invention is an improvement on that of the said patents inthat an improved method of and apparatus for ground noise reduction areprovided.

Ithas been customary in the prior art to operate the noise reductionamplifier, both for variable width and variable density recording, so

that the bias or shutter current from the ground noise reductionamplifier decreases as the modulation increases, reaching zero at somepredetermined modulation level. The reason for this is that the zerocurrent shutter position may be definitely fixed mechanically and formsan interchangeable end stop at maximum modulation.

improved method of and noise reduction. Another object is to provide animproved method of variable density recording.

apparatus for ground Another object of the invention is to provideground noise reduction an improved type of shutter.

Another object of the invention is to provide a recording system whereingroundnoisereduc- If the prior art systems were operated in the reversedirection, that is, if the noise reduction current were to rise withincreased modulation, it would be difficult to rent at a predeterminedThe systems of the prior art had the disadvantage that thecharacteristic curve of the vacuum tube amplifiers usually becomes lessand less linear as it approaches zero current, and it has heretoforebeen difiicult for that reason to establish a definite end value of theshutter opening with increased modulation andat the same time to securea linear response of the ground noise: reductionmechanism in accordancewith modulation.

In our improved apparatus for and method of ground noise reduction, weaccomplish both of the foregoing desirable results simultaneously byincreasing the bias current with increased modulation and providingoptical means for determining the end position of the shutter movement.

suddenly limit the curand unchanging value.

One object of the invention is to provide an tion current increasescreases. i Another objectcof the invention is to prov1de a recordingsystem wherein the ground noise" 7 reduction shutter response has linearrelation to the amplitude of modulation,

Other and incidental objects of the invention will be apparent to thoseskilled in the art from a reading of the specification and theaccompanying drawing, in which Figure 1 is a perspective viewof theimproved recording optical system,

Figure 2 is a schematic wiring diagram of the apparatus,

Figure 3 shows the relation between the, recording beam and the slit atthe position of maximum modulation, and

Figure 4 shows the relation cording beam and the slit at. minimummodulation.

Referring first to Fig. 1, the usual exciter lamp [0 directs a beam oflight past the penumbra mask I i-|2, as in Patent 2,095,317, through thecondenser lens 13, the aperture IS in the between the re the position ofrents passing through the coil 24 Light reflected from the mirror I! isdirected by the, lens 18 to the slit plate IS, the lenses [6 and 18 to-40 gether forming an image of the aperture IE on the slit plate IS in20. Due to the eifect of the mask H, a beam of light graduated inintensity, as indicated at is formed around the slit 2B in the plate l9as the modulation in proper relation to the slit tude of the soundimpulses from the amplifier 21 increase, the output from rectifier, thefilter, and the variable current through the coil 25 increase.

When the current through the coil 25 increases,

the shutter ll, Fig. 1, is moved downwardly, thereby causing light ofgreaterintensity to strike the slit 20 at the same position of the beam.As shown in Fig. 4, when the mask H is in the zero current position withno modulation, the portion of the beam 2! of least intensity coincideswith the slit producing a minimum density on the record 23 and acorrespondingly great density on a print thereof, with correspondinglylow ground noise. When the shutter H is withdrawn, a portion of the beam2| of intermediate intensity, i. e., 50% of maximum intensity, falls onthe slit 20, as shown in Fig. 3, when the galvanometer mirror I! is inthe same position it would occupy if there were no modulation. Due tothe fact that no current may flow through the coil 25 from theamplifienrectifier and filter 28 over this range, the current may be sochosen as to fall on the linear portion of the characteristiccurve ofthe vacuum tubes used, and a correspondingly linear response of theshutter ll may be secured. Likewise, if the shutter II is so constructedmechanically or the magnet 25 is so constructed that the response of theshutter II is not linearv with current through the magnet 25, then anappropriate portion of the characteristiccurve of the output tubes ofthe unit 28 maybe chosen to compensate for this lack of linearity and aresponse of the shutter II which is linear with input from the unit 21to the unit 28 may bethereby secured.

In order to establish a definite end point to the effect of movement ofthe shutter l I, anauxiliary'stop l2 may be provided. It will beapparent thatwhen the upper edge of the shutter l I is past the upperedge of the-stop l2, no furthat our invention is not limited to the formshown in Fig. 1, but that, if desired, duplicate shutters may be usedconnected as shown and described and operated in opposition in themanner shown in Fig. 7 of Dimmick Patent No. 2,095,318, each of theseshutters being provided with an appropriate stop either optical ormechanical.

It will also be apparentthat the stops 1 I and I2 do not need to belocated between the lamp Ill and the lens 13. Either or both of thesestops may be located, if desired, between lens l3 and lens 16, as shownin the aforesaid Patent No. 2,095,318. If desired, one of the stops maybe located; where shown and the other located between the lenses l3 andI6 so that the two stops are optically coincident, or the penumbrasproduced by the two stops at the slit plate l9 are of the same height.

We claim as our invention:

1. Sound recording apparatus including means for directing a beam oflight in the form of a fine line. upon a sensitive film, means formodulating said beam in accordance with sound impulses to be recorded,and means for modulating said beam substantially entirely in a linearrelation to the envelope of the sound impulses to be recorded.

2. Sound recording apparatus including means for directing a beam oflight to a slit, means for imaging light passing through said slit inthe form of a fine line upon a sensitive surface, means for modulatingsaid beam of light in accordance with sound impulses to be recorded, andmeans for further and simultaneously modulating said beam of lightsubstantially entirely in a linear ther effect is produced by furthermovement of the shutter II. The variable intensity light beam remainsfixed in the position shown in Fig. 3. Alternatively, a positivemechanical stop may be provided for the shutter .H precluding itsmovement beyond a position corresponding to the position of the lightstop 12. Further increase of current through the magnet does not produceany further deflection of the shutterll. It will be apparent that thealternative constructions referredto above provide a stop for themovement of this shutter, which stop is either optical, as shown in Fig.1, or mechanical, both producing identical results in that the effectivewithdrawal of the penumbra mask is positively limited and cannot'exceeda predetermined value on increase in ground noise reduction current.

It will be apparent to those skilled in the art relation to the envelopeof said sound impulses. 3. Sound recording apparatus including means fordirecting a beam of light to a slit, means for imaging light passingthrough said slit in the form of a fine line upon a sensitive surface,

means for modulating said beam of light in ac cordance with soundimpulses to be recorded, and

shutter means for further and simultaneously modulating saidbeamof lightsubstantially entirely in a linear relation to the envelope of saidsound impulses.

4. Sound recording apparatus of the penumbra type, including a penumbramask movable in accordance with the envelope of the sound waves to berecorded, a fixed stop located at a position corresponding to themaximum amplitude of the sound waves to be recorded, and means formoving said movable mask substantially entirely in direct linearrelationto the envelope of sound impulses.

5. In sound recording apparatus of the penumbra type, a movable penumbramask, means for moving said mask substantially entirely in linearaccordance with the envelope of the sound waves to be recorded, andmeans for limiting the effective movement of saidmask to correspond withthe maximum amplitude of sound waves'to be recorded.

JOSEPH L. UNDERHILL.

GLENN L. DIMMICK.

